1. Field of the Invention
This invention relates generally to a circuit for controlling gain in low noise amplifiers (LNA""s) or in preamplifiers, and more specifically to gain control circuitry for use in LNAs or preamplifiers used in wireless communication systems.
2. Background of the Related Art
In wireless communication, the incident signal at a terminal unit (e.g. an handset) has various magnitudes, depending on the environment of the radio-wave propagation (including the distance between the terminal and the base station). A strongly radiated signal from a transmitting terminal is helpful to a remote station, but is of less use if the station is nearby. Such a strong signal may impose unwanted spurious signals on foreign terminals near the transmitter, and waste the transmitter""s battery. Additionally, the circuit for amplification in a receiving terminal may clip and distort its output signal for a large magnitude input.
In general, the inward and the outward signal in a terminal are amplified by a low-noise amplifier (LNA) in the receiver block and a preamplifier in the transmitter block, respectively. Good linearity must be achieved in order to suppress the distortion in the next stages. To prevent the LNA from clipping its output signal for a large input, the LNA""s gain has to be made controllable (the gain being lowered for large input signal). Similarly, the gain of a transmitter""s preamplifier must be controlled so as not to waste the battery and also not to overdrive its next stage, the power amplifier.
FIG. 1 shows a related art low noise amplifier that employs current-splitting gain-control techniques. In related art gain-control schemes, the LNA""s gain is decreased by reducing the drain bias current (i.e., by decreasing the gate bias voltage) of its subcircuit responsible for the gain, i.e., a differential-amplifier stage. If the magnitude of an input signal exceeds some prescribed level, then the LNA is entirely bypassed so that no amplification occurs. Unfortunately, the reduction of the bias current is accompanied by degradation of linearity caused by an unwanted cutoff of the driving transistor. An abrupt change of LNA""s gain inherent in the bypass operation makes it hard to simultaneously achieve good linearity and the desired noise performance. Accordingly, a new approach is desirable to control the gain.
An object of the invention is to solve at least the above problems and/or disadvantages and to provide at least the advantages described hereinafter.
Another object of the invention is to provide a method and apparatus for controlling gain in a low-noise amplifier or in a transmitter""s preamplifier.
Another object of the invention is to provide a low-noise amplifier and method of operating same that reduces or avoids clipping its output signal when receiving a large input signal.
Another object of the invention is to provide a method and apparatus capable of controlling the gain of a transmitter""s preamplifier so as not to waste the battery and not to overdrive the next stage (the power amplifier).
In a preferred embodiment of the invention, a variable gain amplifier includes, at least in part, a first transistor having a control electrode, a first electrode, a second electrode and a drain, wherein an input signal is coupled to the control electrode, a first reference voltage is coupled to the first electrode and an output signal is coupled to the second electrode; a load inductor coupled between a second reference voltage and the second electrode of the first transistor; a load capacitor coupled to the second electrode of the first transistor; and a variable resistor coupled in parallel to the load inductor.
To further achieve at least the above objects in a whole or in part and in accordance with purposes of the present invention, as embodied and broadly described herein, there is provided a variable gain amplifier that includes a driving circuit coupled between a first reference voltage and an output terminal, wherein the driving circuit receives an input signal at a control input, a load inductor coupled between a second reference voltage and the output terminal, a load capacitor coupled between the first reference voltage and the output terminal, and a resistor coupled in parallel to the load inductor, wherein the driving circuit has a variable conductance.
To further achieve at least the above objects in a whole or in part and in accordance with purposes of the present invention, as embodied and broadly described herein, there is provided a variable gain amplifier that includes transistor device for amplifying an input signal, wherein said transistor device includes a control electrode, a first electrode and a second electrode, wherein an input signal is coupled to the control electrode, a first reference voltage is coupled to the first electrode and an output signal is coupled to the second electrode, capacitor device for providing capacitance at an output node, wherein the capacitor device is coupled to the second electrode of the transistor device, inductor device coupled between a second reference voltage and the second electrode of the transistor device for resonating with the capacitance device, and variable resistor means coupled in parallel to the inductor device for varying a gain of the amplifier.
To further achieve at least the above objects in a whole or in part and in accordance with purposes of the present invention, as embodied and broadly described herein, there is provided a method of operating a variable gain amplifier that includes receiving an input signal at an input terminal, amplifying the input signal using a driving circuit, tuning an amplifier gain using a parallel-resonant circuit that includes an inductance L and a capacitance C coupled between the driving circuit and an output terminal that outputs an output signal, wherein the parallel-resonant circuit receives the input signal from the driving circuit, controlling the amplifier gain of the output signal by varying a resistance R used in determining a quality factor Q at a resonant frequency of the parallel-resonant circuit, and outputting the gain controlled output signal.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.